Does the time difference between Sydney and Toronto change during the year?

Yes. Because of daylight saving time the gap moves between +14h and +16h.

When is the best time to call Toronto from Sydney?

There is no overlap between standard working days, so one side has to call early or late.

Sydney is 14h ahead of Toronto. This gap is not fixed: daylight saving time moves it between +14h and +16h during the year.

Sydney

Australia

--:--:--

Toronto

Canada

--:--:--

A 9-to-5 working day in Sydney does not overlap one in Toronto at all — somebody has to take the call outside office hours.

City	Time zone	UTC offset
Sydney · Australia	Australia/Sydney	UTC+10
Toronto · Canada	America/Toronto	UTC−4

What is the time difference between Sydney and Toronto?: Sydney is 14h ahead of Toronto. This gap is not fixed: daylight saving time moves it between +14h and +16h during the year.
Does the time difference between Sydney and Toronto change during the year?: Yes. Because of daylight saving time the gap moves between +14h and +16h.
When is the best time to call Toronto from Sydney?: There is no overlap between standard working days, so one side has to call early or late.